1. Field of the Invention
The present application relates to an image processing apparatus, an image processing method, and an image printing apparatus.
2. Description of the Related Art
An ink jet printing method has been known which forms an image on a printing medium by repeating a sequence of scanning operation for printing (hereinafter referred to as print scanning operation) and sub-scanning operation for conveying the printing medium. For the print scanning operation, a printing head ejects ink through a plurality of ejection openings arranged therein while scanning the printing medium.
In such an ink jet printing method, in general, a plurality of ink droplets are ejected to different positions apart from each other by each scanning operation, from the viewpoint of reducing the graininess of the printed image. US 2007/0097164 discloses a printing method using mask patterns the print permitting pixel patterns of which have less low-frequency components than high-frequency components as mask patterns determining the printing positions in a plurality of scanning operations.
Images printed by this ink jet printing method, unfortunately, can exhibit low rub fastness. US 2009/0315935 discloses a technique for ejecting an ink not containing polymer and an ink containing a polymer in that order for each of a plurality of images on a printing medium. In this technique, the ink containing a polymer forms the uppermost ink layer of each image, so that the degradation of the rub fastness of the image can be minimized.
In recent years, ink jet printing methods have been increasingly used to produce printed articles for various applications, and various types of ink and printing medium have been being used accordingly. US 2012/0050388 discloses a method for fixing an image on a printing medium by using an ink containing a polymer emulsion and a water-soluble organic solvent and a less absorbent printing medium. In this method, the ink dropped on the printing medium is heated so that the polymer emulsion coalesces into a film on the surface of the printing medium, thereby fixing the image.
The present inventors however has found that some of the inks containing a polymer emulsion and a water-soluble organic solvent do not produce images having a desired rub fastness in some cases.
This disadvantage will now be described in detail. FIGS. 1A to 1C show a process of fixing of an ink droplet having a relatively high dynamic surface tension having dropped on a printing medium. FIGS. 2A to 2C show a process of fixing of an ink droplet having a relatively low dynamic surface tension dropped on a printing medium.
The term dynamic surface tension refers to a value of the surface tension of a liquid, varying with time, in a dynamic state immediately after the interface thereof has been formed, and is a surface tension in a very short time.
On being applied on a printing medium, an ink is in a dynamic state. FIG. 1A shows an ink droplet 50 having a relatively high dynamic surface tension immediately after being applied on a printing medium. The ink droplet 50 contains a pigment 52 and a polymer emulsion 53. In this state, the ink droplet 50 is heated to evaporate the solvent 54. The solvent 54 contains a water-soluble organic solvent that functions as a film forming agent to reduce the temperature at which the polymer emulsion 53 starts coalescing into a film (minimum film forming temperature).
FIG. 1B shows the ink droplet 50 when the polymer emulsion 53 starts coalescing into a film. Almost all the water in the solvent 54 has been evaporated at this stage, so that the water-soluble organic solvent 55 accounts for a major part of the solvent.
FIG. 1C shows the ink droplet 50 when it is completely fixed to the printing medium 3 by being further heated until the particles of the polymer emulsion are fused. Since the polymer emulsion has completed coalescing into a film at this stage, the pigment particles 52 are covered with the film of the polymer emulsion, thereby forming a flat ink layer 56.
FIG. 2A shows an ink droplet 51 having a relatively low dynamic surface tension immediately after being applied on a printing medium. Since inks are in a dynamic state immediately after being applied on a printing medium, as described above, an ink having a relatively low dynamic surface tension is more likely to spread over the surface of the printing medium and wet it than inks having higher dynamic surface tensions. Accordingly, the surface area of the ink droplet 51 having a relatively low dynamic surface tension becomes larger than that of the ink droplet 50 having a relatively high dynamic surface tension. Consequently, the solvent 54 in the ink droplet 51 is more rapidly evaporated.
FIG. 2B shows the ink droplet 51 having a relatively low dynamic surface tension after a period of time has elapsed which is equal to the time until the ink droplet 50 having a relatively high dynamic surface tension is brought into the stage shown in FIG. 1B by further heating. Since the solvent 54 of the ink droplet 51 can be rapidly evaporated as described above, almost all the water-soluble organic solvent 55 intended to reduce the minimum film forming temperature of the polymer emulsion 53 in the ink droplet, as well as the water, has been evaporated at the stage of FIG. 2B. Accordingly, by the time when the temperature of the polymer emulsion increases to the same temperature as the temperature at the stage shown in FIG. 1A, almost all the water-soluble organic solvent 55 is lost. The ink droplet 51 at the stage shown in FIG. 2B cannot therefore reach the minimum film forming temperature, and the polymer emulsion 53 cannot start coalescing into a film.
FIG. 2C shows the ink droplet 51 after a period of time has elapsed which is equal to the time until the ink droplet 50 is brought into the stage shown in FIG. 1C by further heating. In an ink having a relatively low dynamic surface tension, the film formation of the polymer emulsion is delayed. Thus, the film 57 of the polymer emulsion of the resulting ink layer is not satisfactory, as shown in FIG. 2C. In the region where such an unsatisfactory film of the polymer emulsion is formed, the rub fastness of the ink layer is so insufficient as the image is much likely to separate from the printing medium if rubbed. As such a region increases in an image on a printing medium, the rub fastness of the image decreases.